Color-plus-clear coating systems formed from the application of a transparent topcoat over a colored basecoat have become increasingly popular in the coating industry, particularly for use in coating automobiles. The most economically attractive color-plus-clear systems are those in which the clear coat composition can be applied directly over the uncured colored base coat. The process of applying one layer of a coating before the previous layer is cured, then simultaneously curing both layers, is referred to as a wet-on-wet (“WOW”) application. Color-plus-clear coating systems suitable for WOW application provide a substantial energy cost savings advantage.
Over the past decade, there has been an effort to reduce atmospheric pollution caused by volatile solvents which are emitted during the painting process. However, it is often difficult to achieve high quality, smooth coating finishes, particularly clear coating finishes, such as are required in the automotive industry, without including organic solvents which contribute greatly to flow and leveling of a coating. Organic solvents also can be selected to control rheology of the coating as it is applied, particularly on vertical surfaces. By contrast, water-based coatings which contain little or no organic solvent can exhibit “sagging” or running upon application especially in a high humidity environment. Moreover, in addition to achieving near-flawless appearance, automotive coatings must be durable and chip resistant, yet economical and easy to apply.
Recently, to facilitate throughput in some automotive assembly plants, manufacturers have demanded that the primer-surfacer coating be applied in one coat, as opposed to the conventional two-coat application. The one-coat application can simplify the coating process and reduce capital investment by elimination of a spray booth and related equipment from the coating line. However, in a one-coat application, to achieve desired film-thicknesses, the primer coating composition must be applied using a high fluid delivery rate. When a water-based primer is applied at these high fluid delivery rates, particularly in a high humidity environment (e.g., 75% relative humidity at 75° F. (23.9° C.)), a very wet film usually is obtained and sagging or running of the applied wet coating can result.
The use of powder coatings to eliminate the emission of volatile solvents during the painting process has become increasingly attractive. Powder coatings have become quite popular for use in coatings for automotive components, for example, wheels, axle parts, seat frames and the like. Use of powder coatings for clear coats in color-plus-clear systems, however, is somewhat less prevalent for several reasons. First, powder coatings require a different application technology than conventional liquid coating compositions and, thus, require expensive modifications to existing application lines. Also, most automotive topcoating compositions typically are cured at temperatures below 140° C. By contrast, most powder coating formulations require a much higher curing temperature. Further, many powder coating compositions tend to yellow more readily than conventional liquid, coating compositions, and generally result in coatings having a high cured film thickness, often ranging from 60 to 70 microns.
Powder coatings in slurry form for automotive coatings can overcome many of the disadvantages of dry powder coatings, however, powder slurry compositions can be unstable and settle upon storage at temperatures above 20° C. Further, WOW application of powder slurry clear coating compositions over conventional base coats can result in mud-cracking of the system upon curing. See Aktueller Status bei der Pulverlackentwickluna fur die Automobilindustrie am Beispiel fuller und Klarlack, presented by Dr. W. Kries at the 1st International Conference of Car-Body Powder Coatings, Berlin, Germany, Jun. 22-23, 1998, reprinted in Focus on Powder Coatings, The Royal Society of Chemistry, Sep. 2-8, 1998.
Some aqueous dispersions are known to form powder coatings upon application at ambient temperatures. Although applied as conventional waterborne coating compositions, these dispersions form powder coatings at ambient temperature which require a ramped bake prior to undergoing conventional curing conditions in order to effect a coalesced and continuous film on the substrate surface. Also, many waterborne coating compositions contain a substantial amount of organic solvent to provide flow and coalescence of the applied coating.
The automotive industry would derive a significant economic benefit from an essentially organic solvent-free coating composition which meets the stringent automotive appearance and performance requirements, while maintaining ease of application and properties, such as sag and crater resistance. Also, it would be advantageous to provide an organic solvent-free clear coat composition which can be applied by conventional application means over an uncured pigmented base coating composition (i.e., via WOW application) to form a generally continuous film at ambient temperature which provides a cured film free of mud-cracking.